This invention relates generally to the aeration of waste water and more particularly to a method and apparatus for aerating waste water in an improved manner.
Waste water is commonly treated by aerating the treatment basin in which the waste water is contained. In order to achieve overall efficiency in the aeration system, the efficiency of the oxygen transfer to the waste water should be as high as possible. High oxygen transfer efficiency requires that the diffuser media have a large surface area exposed to the waste water, that the diffusers be located near the floor of the basin, and that strong water circulation patterns (velocity) to the surface of the basin be avoided to maximize bubble residence time. The benefits of longer bubble residence in improved efficiency are well documented in the industry.
Tubular diffusers have many advantages over other types of diffusers, but they are also disadvantageous in some respects due primarily to structural limitations and system geometry. In the past, the length of tubular diffusers has been limited to about two feet because the leverage effect on longer diffusers causes excessive stress on the connection between the diffuser and the air lateral that supplies air to it. With longer diffusers, the uplift force and/or flexure of the diffuser from energy applied by moving water can lead to failure that can disable the entire system. Consequently, the practice has been to install tubular diffusers each two feet long along the length of each submerged air lateral, with the diffusers arranged in pairs extending on opposite sides of the lateral pipe. The effect is to provide evenly spaced diffuser assemblies each centered on the lateral piping and each extending approximately two feet on each side of the air lateral. In addition, economics of first cost suggest use of as few laterals as possible with diffusers close together on the lateral. This condition dramatically reduces system efficiency (increases energy). Therefore, the purpose of this invention is to achieve maximum mixing and oxygen transfer efficiency (minimum energy) with uniform or full floor coverage with diffusers.
In order to provide effective coverage of the basin, it is necessary to utilize a relatively large number of air laterals with relatively widely spaced diffusers. The large number of laterals and piping of conventional diffusers adds significantly to the capital costs of the installation. If the diffusers are arranged in uniform rows ,a strong directional water pumpage to the surface is created at each lateral pipe, and this also results in an undesirable roll pattern of the water circulation. The water flows rapidly upwardly above each air lateral, laterally away from the lateral, and then in a roll pattern back downwardly. The resulting pumpage of the water carries the air bubbles quickly to the surface so that their residence time in the waste water is relatively short and the oxygen transfer efficiency is reduced accordingly. In addition to short residence time, the ratio of bubble velocity to water velocity is small, thus reducing shear on the bubble surface and resulting in the reduced oxygen transfer at the bubble/liquid interface.
Because the velocity with which each air bubble moves to the surface is the sum of the bubble rise velocity and the upwardly velocity component of the water in which the bubble is carried, the residence time can be increased by decreasing the water velocity or the pumpage rate. Consequently, the high rate of water movement that results from strong directional pumpage effects is undesirable and reduces the efficiency of the aeration process. However, some water circulation is desirable in order to prevent the settling of biological solids. Thus, in actual practice, it is necessary to effect a compromise between oxygen transfer efficiency and adequate water circulation when using a roll pattern. By using full floor coverage of diffusers, proper distribution of energy produces proper mixing and efficient oxygen transfer.